Reactions of NH2 species with hydrogen and NO on the Pt(100)-(1x1) surface

Article abstract of DOI:10.1021/jp993863r

Imido and amino NH₂ species are supposed to play a role as intermediates for ammonia synthesis. HREELS and temperature-programmed reaction (TPR) spectroscopy were used to study the reactions of the NH_2,ads species with H₂ and NO to determine its role in the mechanism of NO + H₂ reaction on the unreconstructed Pt(100)-(1x1) surface. The NH_2,ads species was produced during the titration of the H_ads layer with NO on the unreconstructed Pt(100)-(1x1) surface at 300 K. The amino species could readily react with NO at ≥ 260 K, yielding water and N₂. At a reaction temperature of 260 K, N₂ could accumulate on the surface as N_ads, which recombines and desorbs at > 300 K. TPR in the saturated coadsorption layer of the amino species and NO_ads previously prepared at 100 K behaved explosively, manifesting itself in the desorption of the reaction products, e.g., N₂ and water as narrow peaks at ~ 370 K and with fwhm ~ 5-10 K. For the unsaturated layer, reaction started at a lower temperature by ~ 80 K and did not exhibit the explosive behavior. The NH_2,ads species played a role of an intermediate for ammonia formation. The addition of a hydrogen atom to NH_2,ads occurred concurrently with the H_ads recombination at 350-400 K. At > 400 K, amino species disproportionated into N₂ and ammonia.

Full text of DOI:10.1021/jp993863r

J. Phys. Chem. B 2000, 104, 4661-4666
4661
Reactions of NH2 Species with Hydrogen and NO on the Pt(100)-(1×1) Surface
M. Yu. Smirnov* and D. Zemlyanov†
BoreskoV Institute of Catalysis, Prosp. Akademika LaVrentieVa 5, NoVosibirsk 630090, Russian Federation
ReceiVed: October 31, 1999; In Final Form: March 7, 2000
The formation of the NH2,ads amino species and its further reactions with hydrogen and NO on the
unreconstructed Pt(100) surface were studied by means of high-resolution electron energy loss spectroscopy
(HREELS) and temperature-programmed reaction (TPR) spectroscopy. The NH2,ads amino species forms during
the reaction between Hads and NO at 300 K. NH2,ads can be oxidized by NO at a temperature higher than 260
K. The reaction at T g 300 K results in the evolution of N2 and water. An essential amount of Nads accumulates
on the surface after the reaction at ∼260 K and desorbs as a low-temperature TPR peak of N2 at 320 K. TPR
in a saturated coadsorption layer of NOads and NH2,ads prepared at 100 K shows an “explosive” behavior,
manifesting itself in the evolution of narrow TPR peaks of N2 and H2O at ∼370 K. TPR in an unsaturated
coadsorption layer of NOads and NH2,ads proceeds at a much lower temperature of <300 K, resulting in the
“nonexplosive” desorption peaks of N2 and H2O. The reaction between the NH2,ads and Hads species in the
temperature range of 350-470 K results in ammonia evolution. At T < 400 K, NH3 forms through the addition
of a hydrogen atom to NH2,ads and competes with the Hads recombination. At higher temperature, >400 K, the
dissociation of NH2,ads serves as a source of hydrogen atoms for the NH2,ads hydrogenation, leading to the
parallel evolution of ammonia along with N2 and H2. The mechanism of the formation of NH2,ads species and
its further reactions with NO and hydrogen on the Pt(100)-(1×1) surface are discussed in detail.
1. Introduction
proximately normal with respect to the surface.14 In the present
work we aimed to study the reactions of the NH2,ads species
The reaction between NO and H2 on platinum-based catalysts
is of great interest due to its practical importance.1-3 The
selectivity to N2, N2O, and NH3 as reaction products depends
strongly on the reaction conditions as well as on the catalyst
structure. Since imido NH and amino NH2 species are supposed
to play a role of intermediates for the ammonia synthesis, these
species have become a subject of numerous studies.4-14 The
NHads and NH2,ads species were found in the course of the
hydrogenation of atomic nitrogen on Rh(100),4,5 Rh(111),6 Pd-
(100),4,5,7 Ru(0001),8,9 Ru(101h0),10 Ni(110),11 and RhPt-
(100).4,5,12 The distribution between NHx,ads species and their
configuration depends strongly on the nature of the metal surface
and reaction conditions. Both the NHads and NH2,ads species were
observed on the Pt(100) surface during the reaction between
NO and H2.13,14 The significant decrease of a work function
observed during the oscillatory and steady-state regimes of the
NO + H2 reaction on Pt(100) was assigned to the formation of
with H2 and NO in order to understand its role in the mechanism
of the NO + H2 reaction on the unreconstructed Pt(100)-(1×1)
surface.
2. Experimental Section
The experiments were performed in a stainless steel chamber
of a VG ADES 400 electron spectrometer (base pressure e 5
× 10-11 mbar). High-resolution electron energy loss (HREEL)
spectra were obtained in the specular direction at an angle of
approximately 35° with respect to the surface normal, using an
EMU 50 monochromatic electron gun and a 150° hemispheric
deflector type electron energy analyzer. The HREELS resolution,
measured as full width at the half-maximum (fwhm) of the
elastic peak, was approximately 80 cm-1 (10 meV) with a
kinetic energy of electrons of approximately 2.5 eV. Temper-
ature-programmed reaction (TPR) spectra were obtained at a
heating rate of 10 K/s by means of a VG QXK 400 quadrupole
mass-spectrometer supplied with a twin cathode assembly and
a channeltron detector. A self-designed processor-controlled
device was interfaced to the spectrometer for the acquisition of
the HREELS and mass spectrometer data and for controlling
temperature.19,20
A Pt single crystal oriented along the (100) direction within
<0.5° was used in the experiments. The sample, spot-welded
between two tantalum wires, could be heated resistively to 1200
K. The temperature of the crystal was measured by means of a
chromel-alumel thermocouple spot-welded to an edge of the
crystal. The cleaning procedure of the Pt(100) surface included
Ar+-etching and annealing cycles in oxygen and in a vacuum.
The clean and well-annealed Pt(100) surface showed a (5 ×
20) LEED (low-energy electron diffraction) pattern correspond-
ing to the hexagonal structure of the surface. The procedures
ammonia NH3,ads.
15 However, one cannot rule out the formation
of the NHads and/or NH2,ads species because they are expected
to cause a decrease of the work function as well. The theoretical
simulations show that including the NHx,ads species into a scheme
of the NO + H2 reaction is necessary for a proper modeling of
the reaction kinetic and a branch ratio of the products.16-18
In our previous paper,14 we have reported a detailed analysis
of the vibrational data relevant to the NH2,ads species, which
forms during the titration of Hads by NO on the unreconstructed
Pt(100)-(1×1) surface at 300 K. The NH2,ads species occupies
a bridge position bonding via a nitrogen atom to two surface
platinum atoms. The molecular plane of the species is ap-
* Corresponding author. E-mail: smirnov@catalysis.nsk.su.
† Present address: Worcester Polytechnic Institute, 100 Institute Rd.,
Worcester, MA 10609. E-mail: dzem@wpi.edu.
10.1021/jp993863r CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/22/2000

4662 J. Phys. Chem. B, Vol. 104, No. 19, 2000
Smirnov and Zemlyanov
increase, and additionally, a weak band at 825 cm-1 and a
shoulder at 3400 cm-1 become detectable due to wagging [ω-
(NH2)] and asymmetric [νas(NH2)] modes, respectively.27 The
procedures described above was used thereafter for the prepara-
tion of a NH2,ads layer. Since the NH2,ads coverage in the original
Hads layer is about 0.01 monolayer (Figure 1A, spectrum a),21
the maximum attainable NH2,ads coverage is estimated to be 0.12
monolayer after NO exposure of 0.8 langmuir (Figure 1A,
spectrum d). The estimation was done assuming a linear
dependence of the δ(NH2) intensity on θNH and gives a
2
reasonable value for θNH because, according to the stoichiom-
2
etry of the reaction
NOads + 4Hads f H2O + NH2,ads
(1)
Figure 1. (A) Titration of the saturated hydrogen adsorption layer on
the Pt(100)-(1×1) surface with NO at 300 K. HREELS spectra taken
from (a) the saturated Hads layer and after subsequent exposure to (b)
0.2, (c) 0.4, (d) 0.8, (e) 1.2, and (f) 2 langmuirs of NO. (B) TPR spectra
of D2, D2O, 15ND3, 15N2, 15NO, O2, and 15N2O taken after exposure of
the saturated Dads layer to 3 langmuirs of 15NO at 300 K.
the NH2,ads coverage cannot exceed 0.15 monolayer for the initial
Hads coverage of 0.6 monolayer.
The amino species starts to disappear after a NO exposure
of 1.2 langmuirs that manifests itself in the strong attenuation
of the bands attributed to the NH2,ads species (Figure 1A,
spectrum e). The consumption of NH2,ads is accompanied by an
appreciable increase in nitrogen partial pressure, implying that
N2 is a product of the reaction. After the NH2,ads species is
completely consumed, two new bands at 370 and 1585 cm-1
appear due to NO adsorbed on the (1×1) surface (Figure 1,
spectrum f). The former band is assigned to either a ν(Pt-NO)
stretching mode or δ(Pt-N-O) deformation mode,28 and the
latter band is a characteristic of a ν(N-O) stretching vibration.28
The absence of a band at ca. 1770 cm-1, which characterizes
NO adsorbed on structural defects,23 points to the fact that the
(1×1) surface does not reconstruct under the titration of the
Hads layer by NO.
of preparation of the unreconstructed (1×1) surface can be found
elsewhere.21 D2 and 15NO were used for the TPR experiments
in order to increase the reliability of the identification of the
products by mass spectrometry. The exposure was measured in
langmuirs; 1 langmuir ≡ 1.33 × 10-6 mbar‚s. The coverage
was measured in monolayers; 1 monolayer is equal to the
number of platinum atoms in the topmost layer of the unre-
constructed (1×1) surface, i.e., 1.28 × 1015 cm-2
.
3. Results
3.1. Formation of NH2,ads and Its Further Reaction with
NO. HREEL spectra shown in Figure 1A were taken after an
exposure of the Hads layer on the Pt(100)-(1×1) surface to NO
at 300 K. The original Hads layer on the Pt(100)-(1×1) surface
was prepared as follows:
•The Pt(100)-(hex) surface was exposed to NO at 300 K up
to saturation in order to lift the (hex) surface reconstruction.22,23
•The NOads layer was heated at 420 K for 1 min. Heating
induces NOads desorption, producing vacant sites inside the
originally dense NOads/(1×1) islands. The vacant sites are
necessary for the activation of the following titration of NOads
by hydrogen.13,24,25
To perform an additional characterization of the adsorption
layer formed after the Hads titration by NO, TPR spectra of D2,
15ND3, D2O, 15N2, 15NO, O2, and 15N2O were taken after an
exposure of the saturated layer of Dads to NO (3 langmuirs) at
300 K, as shown in Figure 1B. The absence of D-containing
species among the desorbing products, excepting a negligible
amount of D2O, supports the conclusion about formation of a
NOads layer. Indeed, the TPR spectra of NO, N2, O2, and N2O
are very similar to those observed after NO adsorption on the
clean Pt(100)-(1×1) surface.29
The reaction between the amino species and NO was studied
at 260 K by HREELS (Figure 2A). HREEL spectrum a taken
at 100 K from the ND2,ads layer produced as described above
(see Figure 1A for details) shows the characteristic modes of
ND2,ads: δ(ND2) scissors at 1085 cm-1, νs(ND2) symmetric
stretching at 2430 cm-1, and poorly resolved νas(ND2) asym-
metric stretching at ca. 2530 cm-1. The ν(NO) band at 1585
cm-1 reflects the adsorption of a small amount of NO from the
background upon cooling. The HREEL spectra b-e in Figure
2A were taken after NO exposure at a partial pressure of 8 ×
10-10 mbar and 260 K. A gradual attenuation of the character-
istic bands of the ND2,ads species demonstrates the reaction
between ND2,ads and impinging NO. As the ND2,ads species is
consumed, the N-D stretching band becomes more symmetric
and shifts toward higher frequencies by ∼35 cm-1. The ν(NO)
band increases in intensity and shifts upward by ∼30 cm-1 due
to an increase of NOads coverage.23,30 The reaction ends with
the complete consumption of the amino species at a NO
exposure of 1.5 langmuirs. Attempts to perform the reaction at
a temperature of 250 K or below failed.
•The as-activated NOads layer was exposed to 25 langmuir
of H2 at 300 K in order to remove NOads and Oads and to form
a Hads layer on the Pt(100)-(1×1) surface.13,24,25
The as-prepared Hads layer contains two states of Hads21 with
a total coverage of 0.6 monolayer.26 A 2-fold bridge state results
in a band at 1200 cm-1 (spectrum a of Figure 1A), whereas a
major state, occupying a 4-fold hollow position, does not show
any bands in the specular direction.21 The presence of the NH2,ads
species in the amount of 0.01 monolayer manifests itself in the
appearance of weak bands at 1450 and 3300 cm-1 due to the
δ(NH2) scissors and νs(NH2) symmetric stretching modes,
respectively. The NH2,ads species forms during the final stage
of preparation of the Hads layer on the unreconstructed (1×1)
surface.21 A removal of NH2,ads by heating to T > 400 K results
in the surface reconstruction from the (1×1) surface structure
to the (hex) one.
An exposure of the Hads layer to NO results in the disap-
pearance of the bridge state of Hads: the band at 1200 cm-1
decreases in an intensity and then disappears (spectra b and c
in Figure 1A). The intensities of the characteristic bands of
TPR spectra taken after the completion of the reaction
between ND2,ads and NO at 260 K exhibits the evolution of only
N2, NO, and O2 with a minor contribution of N2O, as shown in
NH2,ads, such as νs(NH2) at 3300 cm-1 and δ(NH2) at 1450 cm-1
,

Reactions of NH2 Species with Hydrogen and NO
J. Phys. Chem. B, Vol. 104, No. 19, 2000 4663
Figure 2. Reaction of the amino species with nitric oxide on the Pt-
(100)-(1×1) surface at 260 K. (A) HREEL spectra taken from the
ND2,ads layer (a) and after subsequent exposures to (b) 0.3, (c) 0.6, (d)
0.9, and (e) 1.5 langmuirs of NO at 260 K. The procedures of the
preparation of the initial ND2,ads layer is described in the text. To prevent
the reaction during spectral acquisition, the spectra were recorded at
100 K. (B) TPR spectra of D2, D2O, 15ND3, 15N2, 15NO, O2, and 15N2O
taken after the completion of the reaction between 15ND2,ads and 15NO
at 260 K (correspond to spectrum e in panel A).
Figure 4. Preparation of the coadsorption layer of the amino species
and hydrogen on the Pt(100)-(1×1) surface at 300 K. (A) HREEL
spectra taken from (a) the saturated Dads layer, (b) the ND2,ads layer
prepared by exposure of the Dads layer to 0.7 langmuir of NO and (c)
after subsequent exposure to 85 langmuirs of D2. (B) TPR spectra of
D2, 15ND3, and 15N2 taken from the coadsorption layer of 15ND2,ads and
Dads corresponding to HREEL spectrum c in panel A.
case of the saturated coadsorption layer, TPR exhibits an
explosive behavior manifested itself as extremely narrow peaks
of D2O and N2 at ∼370 K with a fwhm of ∼5-10 K (Figure
3A). The explosive reaction is initiated by the desorption of D2
and NO. Ammonia forms during the initiating stage of the
reaction as well. As soon as the explosive reaction starts at 355
K, the rate of the ND3 formation drops down abruptly. ND2,ads
and Dads are completely consumed during the explosive reaction,
whereas an excess of NO desorbs and dissociates in a similar
manner, as was observed for the individual NOads layers on the
Pt(100)-(1×1) surface.29
The explosive reaction does not take place for the low NOads
coverage, as shown in Figure 3B. Water and nitrogen evolve
as two poorly resolved narrow peaks at ∼300 and ∼330 K.
The presence of sufficient number of vacant sites for NOads
dissociation results in a low reaction temperature, which is
slightly below the onset temperature of D2 desorption and ND3
formation.
Figure 3. TPR spectra of D2, 15ND3, D2O, 15N2, 15NO, and O2 taken
after an exposure of the 15ND2ads layer to (A) 3 langmuirs of 15NO
(saturation) and to (B) 0.3 langmuir of 15NO (nonsaturation) at 100 K.
3.2. Reaction between the NH2,ads Species and Hydrogen.
The reaction between the ND2,ads species and deuterium was
studied in a coadsorption layer of ND2,ads and Dads, prepared as
shown in Figure 4A. The ND2,ads layer (spectrum b) was
prepared by the titration of the saturated Dads layer (spectrum
a) by NO. The HREEL spectrum taken from the ND2,ads layer
shows the characteristic modes of ND2,ads species such as δ-
(ND2), νs(ND2), and νas(ND2). A small contribution of NHDads
results in the appearance of a weak band at 1290 cm-1 due to
the δ(NHD) mode.14 A weak band at 1575 cm-1 reflects the
adsorption of a small amount of NO from the background.13,23,28,30
Figure 2B. While the NO, O2, and N2O spectra are similar to
those observed after NO adsorption on the Pt(100)-(1×1)
surface,29 the N2 desorption shows an additional low-temperature
peak at 320 K along with the traditional peak at 460 K due to
the NOads dissociation. The low-temperature desorption peak
of N2 could be attributed to the recombination of Nads atoms
accumulated during the reaction between ND2,ads and NO.
To get additional information on the reaction between the
amino species and NO, the TPR were performed in coadsorption
layers containing ND2,ads and NOads. The ND2,ads layer was
prepared through the titration of the Dads layer by NO at 300
K; the preparation was controlled by HREELS, as shown in
Figure 1A. Since the formation and consumption of amino
species are the concurrent processes, the procedures of ND2,ads
layer preparation do not allow us to avoid the presence of some
amount of Dads in the resulting adsorption layer. NO adsorption
over the ND2,ads layer was performed at 100 K. Figure 3 shows
two sets of TPR spectra of D2, D2O, 15ND3, 15N2, 15NO, and
O2, which were taken after an exposure of the ND2,ads layer to
3 and 0.3 langmuirs of NO. The NO exposure of 3 langmuirs
is sufficient to saturate the surface with adsorbed species,
whereas a significant amount of vacant adsorption sites is
expected to remain after NO exposure of 0.3 langmuir. In the
An exposure of the ND2,ads layer to 85 langmuirs of D2 at PD
2
) 1.5 × 10-7mbar and 300 K does not lead to any significant
changes of the HREEL spectrum, except that the band at 915
cm-1 due to the bridge state of Dads re-appears and the ν(NO)
band disappears. This means that the ND2,ads species does not
convert into ammonia at 300 K even under a great excess of
deuterium.
Ammonia formation is observed during TPR in the coad-
sorption layer of ND2,ads and Dads, as shown in Figure 4B. Along
with ammonia desorbing as a broad peak at 415 K, the evolution
of N2 and D2 is observed. The TPR spectrum of D2 reveals an
intensive peak at 390 K attributed to the recombination of
21,26,31-34
Dads
.
A weak high-temperature shoulder of the D2

4664 J. Phys. Chem. B, Vol. 104, No. 19, 2000
Smirnov and Zemlyanov
formation of NH2,ads (steps 2c and 2d) competes with the fast
production of water (steps 2e and 2f), the amino species can
form only in an essential excess of hydrogen in the adsorption
layer. As shown in Figure 1A, NH2,ads accumulates during the
early stage of the titration as long as the Hads concentration is
sufficiently high. As the Hads concentration decreases with
increasing NO exposure, the NH2,ads oxidation by NOads starts
to overcome the NH2,ads formation. So, the NH2,ads surface
concentration passes through a maximum and then decays to
zero.
The amino species can react with NO at temperatures higher
than 260 K. NH2,ads is supposed to be oxidized by Oads produced
through NOads dissociation. Since Oads demonstrates a high
activity in splitting out hydrogen atoms from molecules such
37-42
as NH3,ads and N2H4,ads
,
the following mechanism of the
Figure 5. Ammonia formation under the heating of the coadsorption
layer containing the amino species and hydrogen on the Pt(100)-(1×1).
(A) HREEL spectra taken from (a) the NH2-H coadsorption layer and
after its heating in a stepwise manner in a vacuum for 1 min at (b) 350
K, (c) 380 K, and (d) 410 K. The spectra were recorded at 300 K. (B)
TPR spectra of D2, 15ND3, and 15N2 taken from the 15ND2-D coadsorp-
tion layer.
reaction between the NH2,ads species and NOads could be
suggested:
NOads + * f Nads + Oads
NH2,ads + Oads f NHads + OHads
NH2,ads + OHads f NHads + H2O + *
NHads + Oads f Nads + OHads
NHads + OHads f Nads + H2O + *
Nads + Nads f N2 + 2*
(3a)
(3b)
(3c)
(3d)
(3e)
(3f)
desorption peak at 465 K coincides with N2 desorption and
results from the decomposition of amino species.
The thermal stability of the NH2,ads (ND2,ads) species was
studied by HREELS and TPR as shown in Figure 5. The NH2,ads
species is characterized by the bands due to the νs(NH2)
symmetric and νas(NH2) asymmetric stretchings at 3300 and
3400 cm-1, the ω(NH2) wagging at 825 cm-1, and δ(NH2)
scissors at 1450 cm-1 14
. The heating at 350 K results in a strong
Reactions 3 include a transient formation of imido species that
has an experimental proof. Really, the band at 2430 cm-1
possessed by ND2,ads is asymmetric due to unequal contributions
of the νs(ND2) and νas(ND2) modes (Figure 2A, spectrum a),
and it becomes more symmetric upon the oxidation of ND2,ads
by NO (Figure 2A, spectra b and c). The formation of the imido
species characterized by the only ν(ND) stretching mode could
explain the symmetrization of this band.
attenuation of the NH2,ads bands. It is remarkable that this
temperature coincides with the onset temperature of ammonia
evolution shown in Figure 5B. Heating at 380 K does not affect
the intensity of the NH2,ads characteristic bands (spectrum c in
Figure 5A). The NH2,ads species disappears completely after
heating at 410 K (spectrum d in Figure 5A). According to the
TPR data, the disappearance of the amino species is ac-
companied by evolution of ND3, N2, and D2 (Figure 5B).
Nads forming in steps 3d and 3e is supposed to recombine,
resulting in N2 evolution (step 3f). Indeed, no evidence was
obtained for Nads accumulation during the reaction at 300 K
(Figure 1). We have found that Nads producing during titration
of NOads by hydrogen on the Pt(100) surface recombines,
yielding N2 at 300 K.24 Furthermore, Nads formed during the
NH3,ads + Oads reaction on Pt(100) starts to recombine even at
∼200 K.37 The reason for such an unusually low temperature
of N2 recombinative desorption is supposed to be explained by
4. Discussion
4.1. Mechanism of the NH2,ads Species Formation and Its
Reaction with NO. As mentioned in the Introduction, the NHads
and NH2,ads species were observed during the hydrogenation of
adsorbed atomic nitrogen on a number of metal surfaces.4-12
The formation of NH2,ads species on the Pt(100)-(1×1) surface
was reported in ref 14 and in the present work during the
reaction between NO and hydrogen at 300 K. The NH2,ads
species is supposed to form through the hydrogenation of Nads
produced by NOads dissociation as follows:
the destabilizing effect of Oads and NOads on Nads.
37 The similar
effect was reported for Rh(111)43 and Ru(0001).44 As demon-
strated in the present work, Nads can survive on the Pt(100)-
(1×1) surface when the NO + H2 reaction proceeds at 260 K:
the peak of N2 at 320 K obtained in TPR after the reaction
could be assigned to the recombination of Nads (Figure 2B).
As it follows from reactions 3, the vacant adsorption sites
NO + * f NOads
(2a)
(2b)
(2c)
(2d)
(2e)
(2f)
NOads + * f Nads + Oads
Nads + Hads f NHads + *
NHads + Hads f NH2,ads + *
Oads + Hads f OHads + *
OHads + Hads f H2O + 2*
play an important role in the reaction between NH2,ads and NOads
;
therefore, the variation of the total coverage of adsorbed species
changes the character of TPR (Figure 3). TPR in the saturated
coadsorption layer demonstrates explosive behavior of N2 and
H2O formation. The explosive phenomenon was observed
45
previously on the Pt(100) surfaces for the NOads + Hads and
NOads + COads reactions.45-49 According to the mechanism
suggested in ref 47, NOads dissociation is supposed to be the
rate-determining step of the surface explosion, so that it is
initiated by the releasing of vacant sites necessary for NOads
dissociation. In our case, vacant sites in the saturated ND2-
NO coadsorption layer are released through NO desorption, Dads
Here symbol * denotes a vacant adsorption site on the surface.
Oads formed by NOads dissociation reacts readily with Hads
yielding water,34,35 which desorbs immediately.36 The amino
species results from the addition of two Hads to Nads. Since the

Reactions of NH2 Species with Hydrogen and NO
J. Phys. Chem. B, Vol. 104, No. 19, 2000 4665
recombination, and ND3 formation (Figure 3A). As soon as the
reaction is initiated, the number of vacant sites increases rapidly
due to desorption of the products (i.e., N2 and D2O), causing
an autocatalytic acceleration of the reaction. For the unsaturated
coadsorption layer, the surface explosion phenomenon is not
observed (Figure 3B) because the number of vacant sites is
enough for NOads dissociation at a much lower temperature
before NO and D2 desorption and ND3 formation. D2O is
produced in the course of the reaction of Oads with the ND2,ads
and Dads species and desorbs as a double peak at 300 and 330
K. Nads is formed by NOads dissociation and by splitting out of
deuterium atoms from ND2,ads at the same temperatures;
however, the shapes of TPR spectra of D2O and N2 are different.
The main portion of D2O desorbs at 300 K. This reflects the
fact that the oxidation of deuterium from ND2,ads and a residual
temperatures from the low-temperature peak of D2. The broad
NH3 peak might be a superposition of two peaks, which reflects
two different processes. Indeed, the N2 peak and the high-
temperature D2 peak at ∼465 K are likely to result from the
decomposition of the NH2,ads species as follows:
NH2,ads + 2* f Nads + 2Hads
Nads + Nads f N2 + 2*
Hads + Hads f H2 + 2*
(5a)
(5b)
(5c)
Hads produced in stage 5a can react with the amino species,
leading to the formation of ammonia at T > 400 K, resulting in
broadening of the desorption peak of ammonia. At T g 400 K
all the original Hads leaves the surface via the recombination or
ammonia formation (4), therefore, the amino species remains
the only source of hydrogen for the further ammonia formation.
So, the amino species disproportionates into ammonia and
nitrogen.
D
ads is practically completed at ca. 300 K. Nads formed partially
recombines, resulting in the TPR peak at 300 K. However, since
the coverage of destabilizing coadsorbates (mainly NOads
)
decreases as the reaction proceeds, the major portion of Nads
survives on the surface and desorbs at 335 K.
Referring to the low-temperature ammonia evolution shown
in Figure 3A, one can propose another route of NH2,ads
consumption, namely, via the formation of NH3 in the NH2,ads
+ Hads (or NH2,ads + NH2,ads) reaction initiated by compression
of the NH2,ads + Hads coadsorption layer under NO adsorption.
However, the comparison of two sets of TPR spectra presented
in panels A and B of Figure 3 evidences that ammonia can be
produced only when the total coverage of adsorbed species is
close to saturation. Otherwise, the amino species reacts with
NOads, yielding N2 and H2O more readily than with Hads (Figure
3B). In the cases when the reaction was monitored by HREELS
(Figures 1A and 2A), the total coverage of adsorbed species is
far from saturation until the full completion of the reaction, and
therefore, ammonia formation is unexpected. Moreover, the
formation of NH3 would lead to the accumulation of some
amount of the NH3,ads species on the surface, at least when the
reaction is performed at 260 K. However, this is not confirmed
by HREELS measurements, showing the absence of a strong
characteristic band of the δs(NH3) umbrella deformation mode
in the region of 1150 cm-1 (900 cm-1 for ND3,ads).38,50
4.2. Mechanism of the Ammonia Formation. As demon-
strated in Figure 4A, the amino species on the Pt(100)-(1×1)
surface does not convert into ammonia at 300 K even in a great
excess of hydrogen. However, ammonia does form during the
heating of the coadsorption layer of the amino species and
hydrogen at T g 350 K (Figures 4B and 5B). The desorption
of ammonia is accompanied by the consumption of the NH2,ads
species (Figure 5). Likely, the addition of the third hydrogen
to the NH2,ads species is the rate-determining step for the
ammonia formation from Nads on the Pt(100)-(1×1) surface.
The same was established for the Nads hydrogenation on Rh-
(111).6 The following steps for the ammonia formation upon
heating could be supposed:
5. Conclusions
1. The NH2,ads amino species is produced during the titration
of the Hads layer with NO on the unreconstructed Pt(100)-(1×1)
surface at 300 K. The amino species can readily react with NO
at T g 260 K, yielding water and nitrogen. At a reaction
temperature of 260 K, nitrogen can accumulate on the surface
as Nads, which recombines and desorbs at T > 300 K.
2. TPR in the saturated coadsorption layer of the amino
species and NOads prepared previously at 100 K shows an
explosive behavior manifesting itself in the desorption of the
reaction products such as N2 and H2O as narrow peaks at ∼370
K and with fwhm ∼5-10 K. For the unsaturated layer the
reaction starts at a lower temperature by ∼80 K and does not
show the explosive behavior.
3. The NH2,ads species plays the role of an intermediate for
the ammonia formation. The addition of a hydrogen atom to
NH2,ads occurs concurrently with the Hads recombination in the
temperature interval of 350-400 K. At temperatures higher than
400 K, amino species disproportionate into N2 and NH3.
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